009??

It also depends on the level of detail you wish to have and how much space you have got for a layout.

I started some years ago in 009 and was very happy with it.
The kits from Parkside are very simple and easy to construct.
Again Parkside do some kits too for locos.

Then I discovered 7mm narrow gauge which is 7mm to the foot but runs on 00 track.
I converted to this a few years back as the detail is much better and has a better presence on the eye.
People like Wrightlines do loads of 7mm narrow gauge kits.

Now that Slaters have moved into the market I am moving up to 16mm Narrow gauge which runs on O gauge track and is superior in detail given by the bigger scale.

Problem is the costs go up, 009 loco is maybe around £50 now. 7mm loco kits start at around £60-70, but 16mm locos are roughly £300 plus. And if you want to run lengthy slate trains you need to think about wagon costs too. But if you go to a swapmeet you may get some secondhand bargains there.

I have been into Narrow gauge for many years so if you want a chat off line from the forum for advice or whatever by all means please email me. I could send you pictures of some of my models to give you some ideas. Plus I maybe selling some which may interest you.

All the best.

Garry